There have been no previously filed related applications, nor any copending applications anywhere in the world.
This invention relates to insertion type fluid flow sensing elements for measuring air or gas flowrates, and more specifically to an improved tube packing extension assembly for use therewith. It is particularly useful in providing flow element tube access ports for use in conjunction with flow measuring devices in air and gas flow applications in large ducts or pipes, and at high temperatures.
The instrumentation and process control industry has recognized the use of the Pitot tube as a reliable device for measuring the volumetric flow of both liquids and gasses for many years. The Pitot tube operates based upon the principal that when a fixed probe is inserted into piping or duct work containing a moving fluid, the total pressure sensed by the probe is the sum of the static pressure exerted by the fluid, whether in motion or at rest, and the dynamic pressure equivalent to the kinetic energy of the fluid in motion. Conventional Pitot tube arrangements provide measurement of both the static and total pressure of the flowing fluid, the difference between which is the dynamic pressure. This differential pressure, i.e. the dynamic pressure, is directly related to and can be used to calculate the linear flow rate within the piping or duct work. The volumetric flow rate of the fluid is determined by multiplying the linear flow rate by the cross-sectional area of the conduit.
The Pitot tube is particularly useful in measuring gas flows in piping or duct work with a large cross-sectional area because they cause negligible pressure loss within the conduit. For use in conjunction with such traversing Pitot tubes, exterior instrument taps, connected to averaging headers, sensing heads, or the like, are provided for connection of each sensing tube to a differential pressure instrument for indicating flow rate and/or transmitting a flow rate signal. Such instrument taps typically penetrate the duct housing and are affixed to the housing utilizing a ferrule encircling the tube and compressed against a fitting by a nut. Similarly, the opposite end of the sensing tube can be affixed at a manifold or opposing duct housing penetration utilizing compression tube fittings. Such manner of generally affixing tubes penetrating a shell or connecting to other fittings is rather well known throughout the chemical process industries, but can result in material stress and fatigue related leakage or failure if various materials, such as differing grades of steel or stainless steel are utilized, causing differing expansion rates result upon heating or cooling. This is due to the fact that the use of conventional ferrules xe2x80x9clockxe2x80x9d onto the tubing, thereby preventing the movement necessarily resulting from thermal differential expansion
However, one particular improvement, disclosed in U.S. Pat. No. 5,736,651, issued in the name of Bowers, incorporated herein by reference in its entirety, provides a particular improvement wherein a high-temperature tubing connection for a shell penetration incorporates a high temperature packing that replaces the conventional ferrule. A ribbon packing or packing ring made of any pliable material resistant to high temperatures can be utilized for this packing. Particularly disclosed is the use of a GRAPHOIL ((trademark)) packing material, as manufactured by U-CAR ((trademark)), or similar and equivalent material as being successful in permanently xe2x80x9csealingxe2x80x9d the tube between the nut and the fitting, thereby assuring a high pressure seal such that the housing retains its integrity and remains leak-free even under conditions of extreme temperature or extreme temperature gradient cycling. Such a high temperature packing remains pliable to seal around the tubes, while still permitting lateral motion of the traverse tubes which is encountered when thermal differential expansion occurs. Such an improvement needs to be utilized at least at one end of the sensing tube in order to prevent the stress damaged mentioned above.
While many solutions to the problems associated with high temperature flow measurement are incorporated into the Bowers"" invention of U.S. Pat. No. 5,736,651, that disclosure does not address or anticipate the specific problems solved by the present invention. As such, the present invention is sufficiently novel and non-obvious so as to distinguish it from the prior art, including the present inventor""s own prior art. As the commercial success of such high temperature tube packings result in a greater number of industrial applications, other opportunities for improvement in Pitot tube connections have become apparent for use generally within the utility and industrial markets that otherwise have been unapparent. The present invention is one such improvement.
As a result of a general need for providing tube clean out ports in such high temperature Pitot tube applications, the present invention provides a tube packing extension assembly for use in such applications.
According to the preferred embodiment of the present invention, a tube packing extension assembly for use in high temperature gas flow sensing elements and the like is provided. A sensing tube penetrating a duct or conduit housing incorporates a high temperature packing that replaces a traditional ferrule. The high temperature packing is formed of a ribbon packing or packing ring made of any pliable material resistant to high temperatures. However, in its preferred embodiment, a GRAPHOIL ((trademark)) packing material, as manufactured by U-CAR ((trademark)), or similar and equivalent material has been found to be successful in permanently xe2x80x9csealingxe2x80x9d the tube between the nut and the fitting, thereby assuring a high pressure seal such that the housing retains its integrity and remains leak-free even under conditions of extreme temperature or extreme temperature gradient cycling. Such a high temperature packing remains pliable to seal around the sensing tubes, while still permitting lateral motion of traverse tubes which is encountered when thermal differential expansion occurs. Traditionally furnished with such a tube termination is a tube access port assembly having an annular cap fitting terminating the end of the tube and housing a plug fitting for plugging the port thereby created. Such access ports are useful for inspection or cleaning of the array tube interiors should debris accumulate in heavy particulate-laden applications.
In the present invention, the cap fitting with plug fitting is replaced with a tube packing extension enclosure that incorporates an extended, linearly elongated outer tube sheath for circumscribing in a concentric manner any tube end that extends outward from the outer conduit sidewall. This outer tube sheath is formed as attached to a tube fitting connection nut, and provides an extension, of selectable size, that extends outward from the conduit sidewall, yet fully encloses and protects the end of the sensing tube. A tube packing extension cap can thereby be removably affixed to the outer end of the tube packing extension enclosure outward by a sufficient length should any brushing or purging of the tubes be required.